The present invention relates to an apparatus for recording and reproducing digital signals.
In a home-use video signal magnetic recording/reproducing apparatus (hereafter abbreviated as VCR) represented by a cassette VTR which is generally used now, the video signal is subject to frequency modulation (FM) and recorded as slant recording tracks on magnetic tape by rotary heads. The audio signal is recorded as longitudinal tracks on an edge portion of the magnetic tape with high frequency bias by a fixed head.
In recent years, the magnetic tape, head and signal recording techniques have significantly advanced. Accordingly, the recording density has been largely increased. High density recording has been effected amounting to an increase of 17 times as compared with the VTR produced approximately 10 years ago.
These high density recording techniques use reduced tape traveling speed and/or narrowed recording track width. The tape traveling speed is 11 mm/s, for example. And the track width is 20 .mu.m, for example.
In the audio signal recording system using a fixed head, such a low tape traveling speed deteriorates the wow flutter characteristics, the signal-to-noise ratio at the reproduction, reproduction frequency characteristics, or the like, resulting in an insufficient sound quality. Thus, it is difficult to simultaneously satisfy needs of reduction in size, weight and cost and needs of improvement in picture quality and sound quality.
In accordance with one method for making both needs compatible, the audio signal is also converted into an FM signal and recorded onto a video signal recording track by a rotary head. By using this method, the deterioration in the sound quality such as the wow flutter characteristics, signal-to-noise ratio at reproduction and reproduction frequency characteristics is eliminated, and the audio signal is reproduced with high sound quality. In this system, however, the audio signal and the video signal are recorded on an identical part of an identical track. This results in a problem that the after-recording of the audio signal (recording of the audio/video signal onto tape having the information already recorded thereon) is impossible in the editing operation or the like.
Therefore, another method for making the above described needs compatible is described in JP-A-55-32296 (laid open on Mar. 4, 1980). In accordance with this method, the magnetic tape is wound around a rotary cylinder with an angle larger than in the prior art by .theta., and one track is divided into a video signal recording track portion and an audio signal recording track portion (corresponding to the .theta. part). On this audio signal recording track, the time-compressed audio signal is recorded. As a result, it becomes possible to improve the sound quality compatible with the audio dubbing function.
As the application of this method, a VCR having a mode for recording the video signal plus the audio signal and a mode for recording only the audio signal has been proposed in JP-A-58-222402 (laid open on Dec. 24, 1983). In this VCR, the video signal recording track is divided into a plurality of parts. The time-compressed audio signal is recorded onto each of the tracks thus divided. A similar technique is disclosed in U.S. Pat. No. 4,558,378.
In the home-use VCR, it is necessary to prolong the recordable time of the magnetic tape as far as possible and raise the tape usage efficiency. Therefore, it is desirable that the area for recording the above described time-compressed audio signal is made as small as possible.
However, there is a limit in the wavelength which can be recorded on the magnetic tape. In high density home-use recording VCRs now in use, the recordable wavelength is approximately 7 to 8 MHz at most.
On the other hand, the requisites for attaining the high sound quality are as follows:
Dynamic range: 80 dB or more PA0 Frequency band: 15 KHz or more
Assuming that the pulse code modulation (PCM) is used to obtain this sound quality, even audio signal data needs: EQU 32(KHz).times.16(bit).times.2(ch).apprxeq.1.024 Mbits/s
That is, for achieving a 15 KHz frequency band, the frequency at which the audio signal is sampled must be 32 KHz. For achieving an 80 dB dynamic range, the number of bits for quantization of one sampled data or value must be 16 bits, and for stereophonic recording and reproduction, 2 channels (left and right channels) are required. Thus, for audio data, 3,200 (sampling frequency).times.16 (quantization bit number).times.2 (channel number)=1.024 M bits is required per second. Assuming that the time base is compressed to one-sixth (audio signal recording area corresponding to approximately 30.degree. of tape wrap angle; and video signal recording area corresponding to 180.degree. of tape wrap angle), the transmission rate of the audio data required for the recording operation is represented as: EQU 1.024(Mbit/s).times.6=6.144(Mbits/s)
Besides this, it is necessary to record the address data, the error correction data, and the like. The required transmission bit rate of the recording signal depends upon the modulation scheme and is 10 to 15 Mbits/s.
In order to attain the above described improvement of sound quality, after-recording function of the audio signal and high density recording operation, it is indispensable to use a signal recording method facilitating reduction in transmission bit rate and assuring a high sound quality. The above described prior art does not pay attention to these points.